Mars Polar Ice Sample Return (1976-1978)

Image: NASA JPL

Mars, like Earth, has ice caps at its north and south poles. The ice caps on both worlds are dynamic; that is, they expand and contract with the passage of the seasons. On Earth, both the permanent and seasonal polar caps are made up entirely of water ice; on colder Mars, temperatures fall low enough in winter that carbon dioxide condenses out of the atmosphere at the winter pole, depositing a frost layer about a meter thick on the permanent water ice polar cap and surrounding terrain. The three-kilometer-thick permanent caps cover a little more than 1% of Mars’s surface, while the seasonal caps at mid-winter each span from their respective pole to about 60° of latitude.

Confirmation that Mars’s permanent polar caps are made mainly of water ice did not come easily. The polar caps were first glimpsed in the 17th century, and were widely believed to be made of water ice by the end of the 18th. In 1965, however, data from Mariner 4, the first spacecraft to fly past Mars, indicated that the permanent caps were made of frozen carbon dioxide, an interpretation the Mariner 6 and 7 flybys (1969) and the Mariner 9 orbiter (1971-1972) did little to contradict.

In the late 1970s, the Viking orbiters revealed that the northern permanent cap is made of water ice. Confirmation that Mars’s southern permanent cap is also made of frozen water had to wait, however, until 2003, when new data from the Mars Global Surveyor and Mars Odyssey orbiters became available.

Viking orbiter closeup of Mars’s south pole permanent water ice cap at the height of southern hemisphere summer. Image: NASA

In 1976-1977, before the composition of either of Mars’s permanent caps was known with certainty, a team of students in the Purdue University School of Aeronautics and Astronautics studied a Mars Polar Ice Sample Return (MPISR) mission. The mission’s primary goal was to collect and return to Earth a 50-meter-long, five-millimeter-diameter ice core from Mars’s southern permanent cap.

The Purdue team assumed that the polar caps of Mars were, as on Earth, built up of layers of snow or frost deposited anually. Each layer would contain a sample of the dust and gases in the atmosphere at the time it was laid down, making it a record of atmospheric particulates and climate conditions. On Earth, ice cores from Greenland record lead smelting in the Roman Empire and vegetation changes in Ice Age Europe. A martian polar ice core, the students believed, might yield a planet-wide record of dust storms, asteroid impacts, volcanic eruptions, surface water, and the development of microbial life.

Ice core section collected by the Greenland Ice Sheet Project in 1993. This section dates from about 16,250 years ago and covers a span of 38 years. Image: U.S. Geological Survey

MPISR would use a Mars Orbit Rendezvous mission plan similar to that described in a 1974 Martin Marietta/Jet Propulsion Laboratory (JPL) Mars Sample Return (MSR) report. The students envisioned a Viking-derived MPISR spacecraft comprising a 5652-kilogram Mars Orbiter Vehicle (MOV) with “stretched” propellant tanks and a 946-kilogram lander. For comparison, the twin Viking orbiters each weighed only 2336 kilograms at departure from Earth, while the landers they carried to Mars each weighed 571 kilograms. The lone MPISR orbiter would carry a 490-kilogram Earth-Return Vehicle/Earth Orbit Vehicle (ERV/EOV) based on Pioneer 10/Pioneer 11 Jupiter/Saturn flyby spacecraft hardware, and the MPISR lander would include a 327-kilogram Ascent Vehicle (AV) for launching the polar ice sample to Mars orbit.

The MPISR MOV design was derived from that of the twin Viking Mars orbiters, which reached Mars in 1976. Significant modifications would include enlarged propellant tanks and the ERV/EOV for conveying the Mars polar ice sample to Earth. Image: R. Staehle/NASA JPL

The need for a short-duration flight from Mars to Earth and for south pole conditions safe for a lander would dictate the MPISR mission’s Earth departure date. A long flight back to Earth would place great demands on sample refrigeration equipment. Data from the Viking orbiters had shown the south pole ice cap to be too unstable for landing and sample collection in the spring and summer, when the temperature climbs too high for carbon dioxide to remain solid. At mid-winter, on the other hand, snow and frost accumulation might bury the MPISR lander. The team proposed, therefore, that the lander set down 75 days before southern hemisphere autumnal equinox.

The MPISR spacecraft would lift off from Kennedy Space Center, Florida, on 29 April 1986, in the payload bay of a delta-winged, manned Space Shuttle Orbiter. It would reach Earth orbit attached to an expendable Tug derived from the U.S. Air Force/NASA Centaur upper stage. The Purdue students calculated that the proposed Tug could launch up to 9000 kilograms out of Earth orbit toward Mars during the favorable 1986 Earth-Mars transfer opportunity. Their proposed Earth launch approach reflected hopes about the Space Shuttle’s projected capabilities that were not finally dashed until the January 1986 Challenger accident.

On 16 November 1986, after a flight lasting nearly seven months, the MPISR orbiter’s propulsion system would slow the spacecraft so that Mars’s gravity could capture it into a polar orbit. Over the next 14 months, the orbiter would map the martian poles using Viking-type cameras, a Viking-type thermal mapper, and a new-design Radar Ice Sounder for determining ice depth. The sounder, which is not depicted in the MPISR orbiter image above, would employ an 11.47-meter-diameter dish antenna deployed from the orbiter soon after Mars orbit arrival. Scientists on Earth would use data from these instruments to select a safe and scientifically interesting south pole landing site for the MPISR lander.

On 3 February 1988, the lander would separate from the orbiter, ignite solid-propellant rockets to slow down and drop from Mars orbit, then descend through the planet’s thin atmosphere to the selected landing site. Because it would have nearly twice the mass of the Viking lander from which it was derived, the MPISR lander would lower on six parachutes and six terminal descent rocket engines (in each case, twice as many as Viking). The engines would be arranged in three clusters of two engines each.

The Purdue students offered no image of their MPISR lander. Probably it would have resembled this Martin Marietta-designed Mars Sample Return lander based on the company’s Viking lander. Note the modified Viking arm and the barrel-shaped Ascent Vehicle (tipped on its side for easier sample loading). Image: Martin Marietta/NASA

Soon after touchdown, the lander would reach out with its modified Viking sampler arm and detach one of its three descent engine clusters, clearing the way for Ice Core Drill (ICD) deployment. Sixty-seven times over the next 90 days, the ICD would collect a 75-centimeter-long ice core, gradually drilling down to ice and dust layers hidden 50 meters below the surface.

Radioisotope Thermal Generators (RTGs) would power and warm lander systems. The lander’s three footpads and underside would be insulated to prevent its heat from melting the ice, helping to ensure that it would not sink from sight during the three-month sample-collection period.

On 2 May 1988, with winter settling in at Mars’s south pole, the first of the AV’s three rocket stages would ignite to blast the ice core samples into Mars orbit. The first and second stage would burn solid propellants. The liquid-propellant third stage would place the sample container into 2200-kilometer circular orbit about Mars. Refrigeration in the sample container would keep the ice core pristine. The MPISR orbiter would dock with the AV third stage using a docking collar on the ERV/EOV on 17 May, then the sample container would transfer to the ERV/EOV and the AV third stage would be discarded.

On 27 July 1988, the ERV/EOV would separate from the orbiter and fire its engine to leave Mars orbit for Earth. To reduce the period of time the sample container would need to provide refrigeration for the ice core, the ERV/EOV would expend extra propellants to speed its return to Earth. A minimum-energy transfer in the 1988 Mars-Earth transfer opportunity would last 122 days; the ERV/EOV’s energetic Mars departure burn would slash this to 98 days.

Nearing Earth, the cylindrical 1.5-meter-long EOV would separate from the ERV and fire a solid-propellant rocket motor to slow down so that Earth’s gravity could capture it into a 42,200-kilometer circular orbit. The ERV, meanwhile, would speed past Earth into solar orbit.

Discarding the ERV ahead of Earth-orbit capture would reduce EOV mass, thus reducing the quantity of propellant needed to place it into Earth orbit. The Purdue team found that this approach would have mass-saving knock-on effects throughout the MPISR mission design, yielding a 6% reduction in spacecraft mass at Earth launch.

The EOV would carry enough refrigerant to cool the ice sample for 28 days in Earth orbit. During that period, an automated Tug would climb from low-Earth orbit to retrieve the EOV and convey it to a waiting Shuttle Orbiter or an Earth-orbiting space station.

Purdue’s MPISR concept generated considerable interest and demonstrated surprising longevity for a student project. After a summary of the study appeared in the pages of the British Interplanetary Society publication Spaceflight, two of its authors (Staehle and Skinner) briefed JPL engineers on the concept. In 1978, JPL new-hire Staehle pitched a variant of the MPISR plan at a Mars science meeting at the Lunar and Planetary Institute in Houston, Texas.

Elevation map of Mars’s south pole based on data from the Mars Orbiter Laser Altimeter on NASA’s Mars Global Surveyor spacecraft. The permanent water ice cap is the high-altitude brownish region directly above the center of the map. Of only slightly lower altitude, the red region comprises layers of dust laid down by annual accumulation and vaporization of the carbon dioxide ice cap over hundreds of thousands of years. Image: NASA JPL/U.S. Geological Survey